cgroup: rename cgroup_subsys_state->dput_work and its callback function

css (cgroup_subsys_state) will become RCU protected and there will be
two stages which require punting to work item during release.  To
prepare for using the work item for multiple times, rename
css->dput_work to css->destroy_work and css_dput_fn() to
css_free_work_fn() and move work item initialization from css init to
right before the actual usage.

This reorganization doesn't introduce any behavior change.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -80,7 +80,7 @@ struct cgroup_subsys_state {
 	struct css_id __rcu *id;
 
 	/* Used to put @cgroup->dentry on the last css_put() */
-	struct work_struct dput_work;
+	struct work_struct destroy_work;
 };
 
 /* bits in struct cgroup_subsys_state flags field */

kernel/cgroup.c

@@ -4259,10 +4259,10 @@ static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask)
 	return ret;
 }
 
-static void css_dput_fn(struct work_struct *work)
+static void css_free_work_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
-		container_of(work, struct cgroup_subsys_state, dput_work);
+		container_of(work, struct cgroup_subsys_state, destroy_work);
 
 	cgroup_dput(css->cgroup);
 }
@@ -4272,7 +4272,14 @@ static void css_release(struct percpu_ref *ref)
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	schedule_work(&css->dput_work);
+	/*
+	 * css holds an extra ref to @cgrp->dentry which is put on the last
+	 * css_put().  dput() requires process context, which css_put() may
+	 * be called without.  @css->destroy_work will be used to invoke
+	 * dput() asynchronously from css_put().
+	 */
+	INIT_WORK(&css->destroy_work, css_free_work_fn);
+	schedule_work(&css->destroy_work);
 }
 
 static void init_cgroup_css(struct cgroup_subsys_state *css,
@@ -4287,14 +4294,6 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
 		css->flags |= CSS_ROOT;
 	BUG_ON(cgroup_css(cgrp, ss->subsys_id));
 	cgrp->subsys[ss->subsys_id] = css;
-
-	/*
-	 * css holds an extra ref to @cgrp->dentry which is put on the last
-	 * css_put().  dput() requires process context, which css_put() may
-	 * be called without.  @css->dput_work will be used to invoke
-	 * dput() asynchronously from css_put().
-	 */
-	INIT_WORK(&css->dput_work, css_dput_fn);
 }
 
 /* invoke ->css_online() on a new CSS and mark it online if successful */